Ternary oxide BaSnO3 nanoparticles as an efficient electron-transporting layer for planar perovskite solar cells

Accepted Manuscript Ternary oxide BaSnO3 nanoparticles as an efficient electron-transporting layer for planar perovskite solar cells Chongyang Sun, Lin Guan, Yiping Guo, Bijun Fang, Jiaming Yang, Huanan Duan, Yujie Chen, Hua Li, Hezhou Liu PII:

S0925-8388(17)32120-5

DOI:

10.1016/j.jallcom.2017.06.121

Reference:

JALCOM 42190

To appear in:

Journal of Alloys and Compounds

Received Date: 29 March 2017 Revised Date:

8 June 2017

Accepted Date: 11 June 2017

Please cite this article as: C. Sun, L. Guan, Y. Guo, B. Fang, J. Yang, H. Duan, Y. Chen, H. Li, H. Liu, Ternary oxide BaSnO3 nanoparticles as an efficient electron-transporting layer for planar perovskite solar cells, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.06.121. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Ternary oxide BaSnO3 nanoparticles as an efficient electron-transporting layer for planar perovskite solar cells Chongyang Sun a, Lin Guan a, Yiping Guo *, a, Bijun Fang b, Jiaming Yang a, Huanan Duan a, Yujie

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State Key Laboratory of Metal Matrix Composites, School of Materials Science and Engineering,

Shanghai Jiao Tong University. b

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a

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Chen a, Hua Li a and Hezhou Liu *, a

Jiangsu Key Laboratory for Solar Cell Materials and Technology, School of Materials Science

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and Engineering, Changzhou University.

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*Corresponding Author: Yiping Guo (Email: [email protected]); Hezhou Liu (Email:

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[email protected])

Postal Address: Material Building D, Shanghai Jiao Tong University, Dongchuan Road No. 800, Minhang District, Shanghai, 200240, The People’s Republic of China.

ACCEPTED MANUSCRIPT ABSTRACT

Organic-inorganic hybrid perovskites are a promising candidate for fabricating high-efficiency solar cells. In this work, well-dispersed ternary oxide BaSnO3 (BSO) nanoparticles with high

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crystallinity were successfully synthesized by a facile peroxide-precipitate route. And then BSO was first used as an efficient electron-transporting layer in planar perovskite solar cells. Influences

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of the coverage, roughness and thickness of planar BSO layer on the photovoltaic performance of the solar cells were systemically investigated. It is found that better coverage of BSO layer on the

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transparent oxide electrodes can significant reduce current leakage and suitable thickness of electron-transporting layer is favorable to reduce series resistance. Both of them contribute to the improved power conversion efficiency of cell devices. Additionally, the BSO-based device exhibits a comparable device performances to the TiO2 one, which can be ascribed to the efficient

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charge extraction as BSO/perovskite interfaces, the low charge transfer resistance and the suppressed carrier recombination. The optimized BSO-based planar perovskite solar cells exhibits

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power conversion efficiency of 10.96 % with Jsc of 17.45 mA/cm2, Voc of 0.986 V and FF of 0.637. Thus, this work will pave the way for employing ternary oxide as electron-transporting material

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for high-performance perovskite solar cells.

Keywords:

BaSnO3

nanoparticles;

Semiconductors;

Planar

perovskite

solar

cells;

Electron-transporting layer

1. Introduction

Organolead halide hybrid perovskites (CH3NH3PbX3, X=Cl, Br or I) have become a promising photovoltaic materials because of their high absorption coefficient, tunable bandgap, balanced

ACCEPTED MANUSCRIPT charge transport properties with long diffusion length and low temperature processing [1-10]. In 2009, CH3NH3PbI3 perovskites was first used as a light absorber in a conventional dye-sensitized solar cell (DSSC) with only 3.8% efficiency by Miyasaka’s group [1], and the efficiency of this

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kind of perovskite-sensitized solar cell (PSSC) slowly increased to ~6% by Park and his co-workers [2]. However, poor stability and low efficiency in these liquid-electrolyte-based PSSC did not attract much attention. In 2012, breakthrough report [7] on solid-state PSSC with over 9%

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efficiency triggered the research hot on perovskite solar cells. Since then, hybrid perovskites have

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attracted enormous attention, focusing on materials fabrication/device development and a fundamental understanding of materials structural and electronic properties and device operation principles [4, 6, 9]. With these intensive efforts, perovskite solar cells (PSCs) composed of organolead halide hybrid perovskites have made unprecedented progress in just a few years with

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maximum power conversion efficiencies (PCEs) skyrocketing from 3.8% to a certified 22.1% [3]. It has been reported that high-efficiency perovskite cells are intensely dependent on the quality of perovskite films and the perfection of device architectures [6, 9, 11-22]. Today’s state-of-the-art

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PSCs employ various deposition approaches including one-step solution deposition method

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[11-14], two-step sequential deposition techniques [15-17] and vacuum-evaporation deposition process [18, 19] to prepare high-quality perovskite films. In the classical one-step solution process, the perovskite films are directly deposited from the precursor solution. However, it is difficult to obtain a high-quality, uniform and pinholes-free planar perovskite films due to the extremely fast growth rate of perovskites [7, 20]. The most significant advancement in the one-step deposition method is the demonstration of solvent engineering. Cheng et al. and Seok et al. independently reported solvent engineering to prepare uniform, dense perovskite films that lead to an excellent

ACCEPTED MANUSCRIPT photovoltaic performance [21, 22], where the key step for obtaining a high-quality film involves applying an anti-solvent (e.g. chlorine benzene, toluene) during spin coating. This anti-solvent does not dissolve the perovskite precursor films but is miscible with the precursor solvent.

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Recently, this solvent engineering has become an effective technology allowed the reproducible fabrication of high-quality perovskite films in one-step deposition method [5, 6, 10, 13, 14, 23]. In addition, two common device architectures are used for constructing perovskite solar cells, i.e.,

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mesoporous [7, 24, 25] and planar structures [8, 10, 12]. The mesoporous cell structure is adapted

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from solid-state DSSCs, in which the perovskites replace the dye molecules to harvest light [20, 26]. In contrast, the planar cell architecture uses a thin layer of perovskite sandwiched between the hole- and electron-transporting layers such as TiO2 and spiro-MeOTAD, respectively [9]. The planar structure of perovskite solar cells is similar to the conventional thin-film photovoltaic

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technologies. Compared to the mesoporous cell structure, the advantage of such planar PSCs is being free of a high-temperature-processed mesoporous layer, which is expected to be relatively easier for fabricating PSCs [27, 28]. And, most reported planar PSCs have exhibited higher

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photovoltage and photocurrent than mesoporous cells [9, 20, 26, 29].

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In PSCs research community, the binary oxides n-type semiconductors of ZnO, WOx and Nb2O5 have been explored as alternative electron-transporting materials due to their analogy with TiO2 [30-32], but they all exhibited lower device performances. Ternary oxide semiconductors (TOS) are better alternates than binary oxides because of easily tunable optical and electronic properties by altering the composition or doping. Recently, fewer reports are available based on TOS of SrTiO3, Zn2SnO4, and BaSnO3 (BSO) based PSCs [24, 33-36]. However, all of them have been used as mesoporous layers in perovskite solar cells. BSO is a promising candidate due to its

ACCEPTED MANUSCRIPT similar electronic character to TiO2 (band gap and charge transfer kinetics) and its better chemical stability [24, 37-39]. So far, BSO has been widely used in various fields, such as sensors, photocatalysts and dielectrics. There have also been some reports regarding photovoltaic

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application of BSO nanoparticles. Kim et al. [37] reported synthesized-BSO nanoparticles for DSSCs as an efficient photoanode material, and demonstrated PCE of 5.2%. Rajamanickam et al. [40] reported BSO/TiCl4 treated and BSO/scattering layer photoelectrodes in DSSCs can

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effectively increase the photogenerated charge carriers collection, resulting in a superior

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photovoltaic performance. BSO/TiCl4 treated/TiO2 scattering layer photoanode exhibited the highest PCE of 5.68%. Moreover, BSO and La-doped BaSnO3 (LBSO) have also been used as mesoporous scaffolds in PSCs deposited on the hole blocking layers of compact TiO2 film, and exhibited a comparable PCE as well as the mp-TiO2 one [24, 36]. Very recently, LBSO was used

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as an efficient electron transport layer (ETL) in PSCs. Those novel LBSO-based devices exhibit the enhanced PCEs and excellent photostability, which was ascribed to much higher electrical mobility at room temperature and an inferior UV photocatalytic ability than conventional ETL of

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TiO2 [41]. However, as far as we know, BSO has not been solely employed as electron

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transporting layer in planar PSCs yet.

Here, well-dispersed ternary oxide BSO nanoparticles were successfully synthesized by a facile peroxide-precipitate route. BSO nanoparticles were first used as electron-transporting layer in planar PSCs. The correlation between the quality of BSO film and the subsequent photovoltaic performance of cells were investigated in details. Due to the optimized coverage and suitable thickness of BSO layer, the champion planar perovskite solar cell exhibits best PCE of 10.96%, which is, to our knowledge, the highest performance of a BSO-based planar PSCs.

ACCEPTED MANUSCRIPT 2. Experimental

2.1. Fabrication of electron-transporting layer

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Well-dispersed perovskite BaSnO3 nanoparticles (BSO NPs) were synthesized by a facile peroxide-precipitate route, as follows: SnCl4·5H2O (5 mmol, 99%, Aladdin), BaCl2·2H2O (5 mmol, 99.5%, Aladdin) and citric acid monohydrate (2.5 mmol, 99.5%, Aladdin) were dissolved

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in hydrogen peroxide aqueous solution (90 mL, 30%, Aladdin) with constant string, then ammonia solution (25%, Aladdin) was added in the solution to adjust PH value around 10. After stirring for

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12h, the white precipitates were washed by distilled water and ethanol, and were then freeze-dried. The crystallized BSO NPs were obtained by calcining at 800°C for 2 h in air. Then, BSO NPs and TiO2 (commercial Degussa P25) solution (25 mg/mL) were prepared in mixed solvent of

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n-butanol, methanol and chloroform (14:1:1/v:v:v) with ultrasonic assistance, respectively. A volume of 100 µL solution was spin-coated on patterned FTO at 1000 r.p.m. for 9 s and 3000 r.p.m. for 30 s, then dried at 150°C for 10 min. These steps were repeated 1, 2, 3, and 4 times to fabricate

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respectively.

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planar BSO film. Hence, these films were denoted below as BSO-1, BSO-2, BSO-3 and BSO-4,

2.2. Device fabrication

Perovskite

solar

cells

were

fabricated

in

the

following

configuration:

FTO/BSO-x/CH3NH3PbI3/Spiro-OMeTAD/Ag. CH3NH3I (MAI) was synthesized according to our previous work [42]. The CH3NH3PbI3 (MAPbI3) perovskite film was prepared by a previously reported solvent engineering technology [21, 23] with slight modifications of the deposition parameters. The perovskite solution composed of PbI2 (578 mg, 99%, sigma-Aldrich) and

ACCEPTED MANUSCRIPT synthesized CH3NH3I (200 mg) in a mixed solvent (1 mL) of DMF and DMSO (9:1/v: v) was stirred overnight at room temperature in Ar-filled glovebox. Prior to use, the perovskite precursor solution was filtered through a 0.22 µm PTFE syringe filter. The solution was spin-coated on the

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BSO or TiO2 layer at 4000 r.p.m. for 25 s and 200 µL of anhydrous chlorobenzene was quickly dropped onto the center of the rotating substrate before the surface changed to be turbid caused by rapid vaporization of DMF. The obtained films were then annealed at 100°C for 10 min. A volume

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of 35 µL spiro-MeOTAD solution, which composed of 72.3 mg spiro-MeOTAD, 28.8 µL of

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4-tert-butyl pyridine and 17.5 µL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) solution (520 mg/L in acetonitrile ) in 1 mL of anhydrous chlorobenzene, was spin-coated on the perovskite layer at 4000 rpm for 30 s . Finally, a 140 nm thick Ag electrode was deposited by

2.3. Characterizations.

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thermal evaporation to complete the fabrication of the device.

X-ray diffraction (XRD) patterns were obtained from a Rigaku D/MAX 2400 diffractometer

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with Cu Kα radiation operating at 35 kV and 200 mA. The morphology and structure of BSO NPs

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were characterized a transmission electron microscopy (TEM, JEM-2100F, JEOL). Size distribution of the BSO NPs was characterized by zeta sizer (PerkinElmer, 750S). Scanning electron microscopy (SEM, Hitachi S-4800) operating at 10 kV was employed to characterize the morphologies of the films and the cross section of the devices. The Image-Pro Plus software was used to analyze the coverage of BSO NPs on the transparent electrode FTO. The root-mean-square roughness (RMS) and topography images of the perovskite films were measured by atomic force microscopy (AFM, Nanonavi E-Sweep, Seiko) in tapping mode. UV-vis spectra and transmittance spectrum were recorded on the UV-vis spectrophotometer (Agilent, model Cary 60). The J-V

ACCEPTED MANUSCRIPT curves were measured by a Keithley 2400 source meter under AM 1.5G illumination (100 mW/cm2). The light intensity was calibrated with an NREL-calibrated Si solar cell with KG-2 filter. A black metal mask with the area of 0.09 cm2 was used to prevent scattered light during J-V

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measurement. The external quantum efficiency spectra were measured on a Enlitech QE-3011 system. Steady-state photoluminescence (PL) spectra were recorded on a luminescence spectrometer (Model: LS 55, Perkin Elmer). Time-resolved photoluminescence (TR-PL) decay

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spectra were measured on a PTI QM/TM/IM fluorescence spectrometer at room temperature,

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where poly(methyl methacrylate) (PMMA) serves as protective layer for perovskite films. Impedance spectroscopy (IS) was carried out by using impedance/gain-phase analyzer (Solartron SI 1260) at open-circuit potential condition, with the frequency ranging from 100 kHz to 1 Hz and modulation amplitude of 10 mV.

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3. Results and discussion

The XRD patterns and SEM of the synthesized BSO NPs are shown in Fig. 1a and Fig. 1b,

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respectively. All the diffraction peaks are indexed as perovskite BSO (JCPDS 15-0780) without

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other impurity phases, which is consistent with the previous reports [24, 37]. Additionally, the average crystallite size was calculated with Scherrer equation using the full width at half maximum (FWHM) of the strongest (110) diffraction peak and it is found to be ~24 nm. Scherrer equation: D = (K*λ)/(β*cosθ), where D is the crystallite size, K is a constant, λ is the wavelength of Cu Kα radiation, β is the full width at half-maximum(FWHM), and θ the scattering angle. The degree of crystallinity greatly affects charge transport. Therefore, the crystallinity of BSO NPs was further characterized by TEM. The high-resolution TEM (HR-TEM) image (Fig. 2a) and selected area electron diffraction (SAED) pattern (Fig. 2b) reveal that the synthesized

ACCEPTED MANUSCRIPT nanoparticles exhibit a BSO single phase with high crystalline. The lattice TEM image reveals that

the nanoparticle surface is almost perfect and free from defects; the lattice fringe spacing in the HR-TEM lattice image is 2.9 Ȧ, which is in good agreement with the interlayer spacing of (110)

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plane of BSO. And, the particle size is determined to be 21.3 ± 3.5 nm (Fig. 2c), which matches well with the result determined by "Scherrer equation". But, aggregated particles are also observed,

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which could result in the presence of larger particles (over 50 nm). Size distribution of BSO NPs (Fig. 2d) shows that the synthesized BSO NPs are uniform in term of the particulate size. Larger

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particles (over 150 nm) are almost not detected.

It has been reported that BSO is a cubic perovskite-type oxide and behaves as a n-type semiconductor [38, 43], and exhibits a high electron mobility at room temperature [39, 44]. The

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device structure and energy level diagram of the BSO-based PSCs are depicted in Fig. 3 (band bending is not shown). It has been reported that perovskite BSO is an indirect band gap semiconductor with a highly dispersed band structure [37, 45]. For PSCs, the energy level

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matching between the perovskite material and the ETL is critical for the efficient extraction of

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electrons and the matching band structure leads the electron and the hole to separate effectively, which greatly influences the device performance. Alignment between the CB of ETL governs the extraction of photogenerated electrons, and the CB of perovskite layers depend on the different oxides used as ETLs. It is obvious that favorable energy level alignment could be achieved between BSO and MAPbI3. This will lead to efficient carrier extraction and transportation without quasi-Fermi level splitting in the light-harvesting MAPbI3 layer or excessive interface recombination [46]. Additionally, the energy level of BSO matches well with other layers, which lowers the driving force needed for electron injection from MAPbI3 to BSO. However, for

ACCEPTED MANUSCRIPT TiO2-based PSCs, a quite low electron mobility of TiO2 ETLs is a negative factor to improve the device’s performances [36, 47]. Thus, BSO can be used as electron-transporting materials in dye-sensitized solar cells or mesoporous perovskite solar cells [24, 37, 38]. Here, BSO NPs are

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first utilized as an electron transporting layer in planar perovskite solar cells.

With the advantages of small and uniform particle size, BSO NPs are well dispersed in the

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mixed solvent as stated in the experimental section. High coverage and suitable thickness of BSO layer is critical for high-performance BSO-based PSCs. SEM images and normal camera pictures

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of the various BSO films are presented in Fig. 4. The thickness of BSO layers is determined by the cross-sectional SEM images (Fig. 4(a3-d3)). The thickness of BSO-1, BSO-2, BSO-3 and BSO-4 are determined to be around 30, 63, 79 and 123 nm, respectively. In the case of BSO-1, there is a poor coverage on transparent electron FTO (Fig. 4(a2)), where BSO NPs tend to accumulate on

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the junction of grains and further smooth the surface of FTO with decreased roughness of film, as shown in Fig. 5. But, FTO is still partial bare without being covered. The coverage of BSO NPs of

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BSO-1 on FTO, obtained by using Image-Pro Plus software, is estimated to be around 60%. The exposure of FTO, which could directly contact to perovskite, decreases the open-circuit voltage

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(Voc) and further leads to low efficiency. With increasing the number of spin, FTO is well covered by BSO nanoparticles with improved coverage rate and thickness. The BSO film with an almost 100% surface coverage on FTO could be found in BSO-3 and BSO-4 (Fig. 4(b2, c2)), which is verified by AFM topographies (Fig. 5). However, the more increased thickness of BSO-4 films (the number of BSO NPs layer) will increase the roughness of BSO films. The root-mean-square roughness (RMS) of various BSO films are obtained from AFM topographical images. The RMS values of FTO, BSO-1, BSO-2, BSO-3 and BSO-4 are 34.5, 32.5, 28.4, 31.4 and 37.8 nm,

ACCEPTED MANUSCRIPT respectively. The UV-vis spectra of BSO NPs are shown in Fig. 6a. The band-gap of BSO NPs can be obtained from Tauc’s equation calculated by UV-vis spectra: αhν = B(hν − Eg)௡ , where α is the absorbance coefficient, hν is the photon energy, B is a constant, Eg is the band-gap, and n = 2

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as BSO is a indirect band-gap semiconductor. From the Tauc plot (Fig. 6b), the band-gap Eg is determined by extrapolating the linear part of the curve to zero of vertical axis. The band gap of BSO NPs is found to be 3.1 eV, which is coincident with the optical gaps reported in the previous

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literatures [24, 37, 40]. After the application of BSO NPs on the FTO, the transmittance of the

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BSO film increase (Fig. 6c), which is ascribed to the reduction of light reflectance due to the higher light scattering of BSO nanoparticles film than that the pristine FTO [36, 40].

High-quality perovskite deposited on the BSO film has been prepared by a simple one-step spin-coating method and the detailed procedure has been discussed in the Experimental section.

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The typical SEM image of perovskite film deposited on FTO/BSO-3 substrate is displayed in Fig. 7a. Perovskite film exhibits ultra-smooth, uniform surface, pinhole-free and full coverage on the

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BSO film. The average grain sizes of perovskites is determined to be around 200.3 ± 81.1 nm. The XRD patterns of CH3NH3PbI3 perovskite films deposited on the planar BSO film is displayed in

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Fig. 7b. It shows that perovskites exhibit a tetragonal perovskite structure with I4cm symmetry and a high phase purity, which is in good agreement with the reported studies [48]. The UV-vis spectra of perovskite films shows a typical MAPbI3 absorbance [27, 28, 34], as depicted in Fig. 7c. There is no significant difference in light absorbance of MAPbI3 film deposited on various BSO films, which indicates that the light harvesting of MAPbI3 prepared by the mentioned method is independent on the thickness coverage of BSO layers. The transmittance of the perovskite films deposited on BSO films is displayed in Fig. 7d. There is no significant difference of transmittance

ACCEPTED MANUSCRIPT among various perovskites films deposited on different thickness BSO layers in the wavelength region below 780 nm. The transmittance of perovskite film increase in the wavelength region over 780 nm, which can be attributed to the increased light scattering for thick BSO layers [15, 39, 40].

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The typical photocurrent density-voltage (J-V) curves of MAPbI3 devices fabricated by using different BSO layers in the same batch under simulated 1 sun illumination are presented in Fig. 8a.

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Photovoltaic performance parameters including the statistical analysis of all these devices are summarized in Table 1.A correlation between BSO film qualities and the device parameters can be

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deduced, as evident from the substantial improvement in photovoltaic performance in device prepared using a high coverage and suitable thickness of BSO films. PCEs of the cells prepared with BSO layers of BSO-1 and BSO-2 are 0.80 % (along with Jsc=4.39 mA/cm2, Voc=0.596 V and FF=0.306) and 7.93% (along with Jsc=17.94 mA/cm2, Voc=0.880 V and FF=0.502), respectively.

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The poor performance can be ascribed to be the direct contact between perovskite and bare FTO due to the low coverage of BSO film, in which the current leakage of the thin BSO film is

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maximized [4, 42, 49]. The efficiency for BSO-4-based cell is increased to 8.01% with a Jsc of 12.68 mA/cm2, Voc of 1.008 V, and FF of 0.626. The efficiency enhancement is largely determined

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by the increased Voc value. The cells has a higher shunt resistance (Rsh). It indicates that the BSO-4 film has a better hole blocking because the number of shunting paths decreases, leading to larger Voc [35]. However, there is a significant decrease in short-circuit photocurrent (Jsc), which ascribe to the increasing of series resistance (Rs) due to the application of the thicker BSO layer. Taking both Voc and Jsc into account, planar PSCs possess the best photovoltaic performance by employing BSO-3 layer because of its best qualities (high coverage and suitable thickness) as a planar electron-transporting layer for efficient PSCs. The typical cross-sectional SEM images of

ACCEPTED MANUSCRIPT BSO-3-based PSC are presented in Fig. 8b. The thickness of BSO layer, MAPbI3 and hole-transporting layer is around 79 nm, 275 nm and 153 nm, respectively. It also shows that the interfaces between BSO layer and MAPbI3 are not obvious, which indicates fewer crystalline

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boundaries or grainy textures and this could reduce the energy loss and interface resistance [46].The device with BSO-3 exhibits Jsc of 17.45 mA/cm2, Voc of 0.986 V, fill factor (FF) of 0.637 and PCE of 10.96% in our experiment, which is comparable to that of BSO-based mesoporous

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PSCs [24]. To the best of our knowledge, this is the highest reported conversion efficiency of

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BSO-based planar perovskite solar cells. In order to confirm the accuracy of the PCEs results, the external quantum efficiencies (EQEs) for the devices are also compared. The EQE responses (Fig. 8c) of these devices reveal a significant contribution at wavelengths between 400 and 800 nm, which correspond to the absorption spectra of perovskite films. By analyzing the convolution of

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the spectra responses with the photon flux AM 1.5G spectrum, the calculated Jsc of the devices fabricated by using BSO-1, BSO-2, BSO-3 and BSO-4 are 3.16, 16.44. 16.27 and 11.49 mA/cm2, respectively. The integrated Jsc values are consistent with the Jsc value obtained from the J-V

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curves. There is a mismatch of approximately 10% between the convolution and solar simulator

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data, which is due to the mismatch between the EQE spectra and the photon flux AM 1.5 G spectrum and the decay of the devices when the J−V curves and EQE measurements are performed outside the glovebox [28, 48].

To understand the improvement of electron mobility in BSO electron-transporting layer compared to TiO2 film, PCE of BSO NPs and TiO2 based planar PSCs were compared, both devices being fabricated in the same batch by using the above identical experimental procedures. SEM images of perovskites deposited on BSO and TiO2 films are shown in Fig. 9(a1, b1). It can

ACCEPTED MANUSCRIPT be observed that both planar BSO and TiO2 films have a good surface coverage with a MAPbI3 films. The films are well crystallized and there are pinholes-free between crystal boundaries. In contrast, the MAPbI3 films above BSO (RMS=11.4 nm) are more smooth than the TiO2 sample

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(RMS=14.3 nm). Greater surface coverages lead to a better overall contact. The cross-sectional views of planar-BSO and TiO2 based PSCs are displayed in Fig. 9(a3, b3). These two films have a similar total thickness of about 390 nm. Fig. 10a presents the XRD patterns of perovskites

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deposited on TiO2 and BSO films and shows that all MAPbI3 films exhibited a high phase purity.

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As we known, it is effective to assess crystallinity of the samples by the full width at half maximum (FWHM) of strongest diffraction peaks. FWHM of MAPbI3 (110) diffraction peak are extracted from the XRD patterns and shown in Fig. 10a. The similar FWHM of MAPbI3 (110) diffraction peak confirms that no obvious difference in the diffraction peak intensity (indicating

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the similar crystallization). The UV-vis spectra (Fig. 10b) of perovskites deposited on BSO and TiO2 films all show identical broad absorptions. The J-V curves and IPCE spectra of typical BSO and TiO2 based planar PSCs are displayed in Fig. 10c and d. The J-V curves show that the

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BSO-based device can perform as well as the TiO2 sample and even better. The Jsc, Voc and FF

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values of BSO and TiO2 based devices are 16.95 mA/cm2, 0.976 V and 0.629, and 16.29 mA/cm2, 0.979V and 0.587, respectively. Since the light harvesting has a positive proportion to the absorption value, the solar cells with different ETL show on obvious difference in light harvesting according to the above UV-vis measurement results (Fig. 10b). The difference of Jsc between the two cases can be attributed to the relatively high electron mobility of BSO due to fast charge extraction in BSO/MAPbI3 interface and charge carrier transport in ETL [24, 37, 44]. The lower Voc for BSO-based cells is mainly attributed to the lower conduction band level of BSO [37, 38].

ACCEPTED MANUSCRIPT It has been reported that the conduction band edge of TiO2 (-0.5 eV vs NHE) is higher (i.e., more negative) than that of BSO (-0.2 to-0.4 eV vs NHE) [38, 50]. The BSO layer exhibits a conduction band minimum (CBM), which is away to the CBM of MAPbI3. It can make charge carrier

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‘‘transfer back’’ to MAPbI3 more difficult and decrease carrier recombination, and further lead to a high FF for BSO-based PSCs. Additionally, the enhancement of cell performance is mostly consistent with its much longer photoluminescence (PL) carrier lifetime and increased carrier

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recombination resistance, as the below discussed. The integrated current densities of the devices

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fabricated on BSO and TiO2, calculated from the EQE spectra (Fig. 10d), are 15.89 and 15.29 mA/cm2, respectively. The integrated Jsc values are consistent with the Jsc value obtained from the J-V curves. The EQE intensity drop at a wavelength longer than 780 nm, which may be owing to the perovskite film having a lower absorption in the visible spectrum [24, 48].

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It has been reported that the charge extraction efficiency, charge transport and recombination characteristics in solar cells can be investigated by using the photoluminescence (PL) quenching

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and decay lifetime [29, 41, 51], impedance spectroscopy [52-55], intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS)

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[25, 26, 49], and open-circuit voltage decay [53, 56]. On contact with ETLs, perovskite films typically exhibit strong photoluminescence (PL) quenching as evidence of efficient charge extraction at the interface of ETL/MAPbI3 [48, 57]. The PL quenching could be discussed from the following three reasons: (1) the intrinsic radiative recombination of photogenerated electrons back to the ground state or (2) the non-radiative recombination resulting from defects or boundaries and (3) the extraction of electron-transporting layers. In order to study the charge separation and extraction efficiency of BSO and TiO2 as an ETL, the PL quenching of MAPbI3

ACCEPTED MANUSCRIPT emission is investigated. In general, the radiative recombination takes place at a certain rate corresponding to perovskite materials properties. Thus, the faster or slower quenching of PL intensity can be derived from the extraction efficiency or the defect states in the interface of ETL

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and perovskite films. Fig. 11a shows the steady-state PL spectra of the MAPbI3 films on glass/FTO, glass/FTO/TiO2 and glass/FTO/BSO. The PL spectra of MAPbI3 show strong emission peaks at a wavelength of ~775 nm corresponds to the band-edge emission, which is consistent

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with the previously reported results [58, 59]. Obviously, when perovskites deposited on ETL of

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BSO and TiO2 films, the PL intensity of the perovskite significantly decreases, indicating that the carrier extraction is across the interface between MAPbI3 and ETLs [51, 57]; On the other hand, MAPbI3 grown on planar BSO exhibits lower PL intensity compared to that on TiO2, indicating efficient photoelectron transfer from MAPbI3 to BSO. This is because that compared with that of

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TiO2，the deeper CBE of BSO provides a large potential for electron transfer between the perovskite layer and the ETL following the energy gap law. Carrier extraction behavior of different ETLs is further verified by the time-resolved photoluminescence (TR-PL) spectra. Fig.

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11b shows the decay of PL intensity for perovskite on different ETLs. The lifetime (τ) of PL

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signals is thus obtained by exponentially fitting the decay curves. All curves are well fitted with a double-exponential decay function: I(t)=A1*exp(-t/τ1) + A2*exp(-t/τ2), where t is the time after optical excitation, I(t) is the luminescence intensity at time t, A1 and A2 are coefficients, and τ1 and τ2 are the fast and slow decay lifetimes, respectively. The shorter PL lifetime with BSO as the ETL confirms that the photo-induced electron transfer between perovskite and BSO is more efficient than that between perovskite and TiO2 [57]. The charge extraction efficiency is one of the factors influencing the Jsc value in PSCs [60], thus for BSO-based sample, the high quenching efficiency

ACCEPTED MANUSCRIPT with short PL lifetime contributes to a high Jsc value of the corresponding PSCs.

Impedance spectroscopy (IS) can be used to elucidate the charge-transfer properties and charge recombination characteristics in solar cells [53-55, 58]. In order to better investigate the difference

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in the interface charge collection properties of PSCs based on BSO and TiO2 as a ETLs, we carried out IS measurements at open-circuit potential on the devices with the configuration of

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FTO/ETL(BSO or TiO2)/MAPbI3/spiro-MeOTAD/Ag. Fig. 12a shows the Nyquist plots of the corresponding PSCs under 1 sun illumination conditions. Fig 12b shows the equivalent circuit

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model which is employed to fit the experimental data following by the previous investigation [29, 53, 58]. The fitted curve is in good agreement with experimental data. The series resistance (Rs) is equal to the value of high-frequency intercept on the real axis, which is assigned to the resistance of external wires and FTO substrates. The high-frequency feature is ascribed to the charge transfer

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resistance (Rct), which is influenced by transport resistance at the ETL and HTL (spiro-MeOTAD), but also by the charge transfer resistance at the ETL/MAPbI3 and MAPbI3/spiro-MeOTAD

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interfaces [55, 61]. The Rct values of BSO and TiO2-based PSCs obtained from fitting of the equivalent circuit are 212 and 238 Ω, respectively. The BSO-based PSC gives the reduced

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resistance values indicating the charge extraction and transfer becomes more efficient at the BSO/MAPbI3/spiro-MeOTAD interfaces, which is consistent with the above PL analysis. Moreover, the lower Rct value contribute to the high Jsc value of PSCs. Impedance at the low-frequency range corresponding to right incomplete arc reflects the resistance of charge recombination (Rrec)[53, 55]. It is found that BSO-based PSC has a higher Rrec (2857 Ω) than that of TiO2-based PSC (1404 Ω). Since recombination resistance is inversely proportional to the charge

recombination

rate

[53],

the

charge

recombination

is

unfavorable

between

ACCEPTED MANUSCRIPT BSO/MAPbI3/spiro-MeOTAD. Therefore, the higher FF value of BSO-based PSC could be mainly attributed to the suppressed charge recombination between interfaces with a concomitantly low Rct. This is verified by IS measurements under dark conditions (Fig 12c). Fig. 12d shows that the Rrec

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values, derived from the Nyquist plots, as a function of voltage for the two solar cells. The Rrec for both PSCs strongly depends with the bias voltage. They tend to decrease when the bias voltage increased, which is in agreement with previous reports [52, 53, 55]. The Rrec values for the

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BSO-based devices are generally 1-2 orders of magnitude higher than those of the BSO-based

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devices. Thus, the recombination rate for BSO-based device is much lower than that for the BSO-based device. The observed difference in Rrec is consistent with their obviously different dark J-V characteristics (Fig 10c). The onset voltage of the dark current shifts from about 480 mV for the TiO2-based device to over 720 mV for the BSO-based device. Both Rrec and dark J-V results

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indicate the suppression of the charge recombination for the BSO-based device. As a result, the BSO-based PSCs exhibits a low Rct and a high Rrec, which indicates its high charge transfer rate and low carrier recombination rate, leading to the enhanced photovoltaic performance. We confirm

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4. Conclusions

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this by TR-PL measurements. Therefore, BSO is a promising ETL in planar perovskite solar cells.

In summary, we have demonstrated the first use of ternary oxide BSO NPs as the planar electron-transporting layer in PSCs. The photovoltaic performances of solar cells are significantly improved by optimizing the qualities (coverage, roughness and thickness) of BSO layers. Our results reveals that, BSO-3 based planar PSCs exhibits the best PCE with the better Jsc, Voc and FF due to the suitable thickness of BSO layer with the improved surface coverage on FTO, reduced current leakage and decreased charge transfer resistance. Compared with the control TiO2-based

ACCEPTED MANUSCRIPT cell device, an improvement (nearly 12%) in the PCE is obtained for BSO-based PSCs. Steady-state PL and PL lifetime studies show that the photo-induced electron transfer between perovskite and BSO is more efficient than that between perovskite and TiO2. Analyses of

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impedance spectra reveal that BSO-based PSC exhibits the suppressed charge recombination and a low Rct comparing with TiO2-based PSCs. All of efficient charge extraction and lower charge transfer resistance and reduce carrier recombination leads to the enhancement of corresponding

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device performance. Thus, the new BSO-based planar PSCs will be a competitive candidate in the

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future by further optimizing the preparation procedure of perovskite film and interfaces of cell devices, and this work will pave the way for employing ternary oxide as electron-transporting material for high-performance perovskite solar cells.

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Acknowledgements

This work is supported by the National Natural Science Foundation of China (No. 11474199) and Research Fund of Jiangsu Key Laboratory for Solar Cell Materials and Technology (No.

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SKLPSTKF201501). Instrumental Analysis Center of Shanghai Jiao Tong University and National

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Engineering Research Center for Nanotechnology are sincerely acknowledged for assisting relevant analyses.

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ACCEPTED MANUSCRIPT Figure captions

Fig. 1. XRD pattern (a), typical SEM image (b) of the synthesized BSO nanoparticles. Inset shows

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crystallite size calculated by Scherrer equation.

Fig. 2. High-resolution TEM image (a), selected area electron diffraction (SAED) pattern (b), low-resolution TEM image (c) and size distribution (d) of the synthesized BSO nanoparticles.

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Inserted table shows size of BSO NPs obtained from TEM and Scherrer equation.

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Fig. 3. Device structure of the perovskite solar cells with BSO ETL (the HTL is Spiro-OMeTAD), (b) the schematic energy level diagram of the BSO-based device.

Fig. 4. Typical SEM images of planar BSO-1 (a), BSO-2 (b), BSO-3 (c) and BSO-4 (d) film; the low magnification images (1), magnified images (2), and cross-sectional SEM images (3) of the

bars are 500 nm.

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corresponding BSO film. Insets show the normal camera pictures (e) of various BSO films. Scale

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Fig. 5. AFM topographical images (2µm x 2 µm) of the various BSO films and pristine FTO

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substrates; pristine FTO substrates (a), BSO-1 (b), BSO-2 (c), BSO-3 (d) and BSO-4 (e) film.

Fig. 6. UV-vis absorption spectra (a) and Tauc’s plot (b) of BSO nanoparticles; the total transmittance (c) of the various BSO films.

Fig. 7. Typical SEM image (a) and XRD pattern (b) of MAPbI3 film deposited on planar BSO film; UV-vis absorption spectra (c) and the total transmittance (d) of MAPbI3 films deposited on various BSO films.

Fig. 8. Typical J-V curves (a), cross-sectional SEM image (b) and external quantum efficiency

ACCEPTED MANUSCRIPT (EQE) (c) of BSO-based planar perovskite solar cells.

Fig. 9. SEM images (1), AFM topographical images (2) and cross-sectional SEM images (3) of

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perovskites deposited on BSO films (a) and TiO2 films (b), respectively. Scale bars are 500 nm.

Fig. 10. XRD patterns (a) and UV-vis absorption spectra (b) of perovskites deposited on BSO and TiO2 films, respectively; Typical J-V curves (c) and EQE spectra (d) of planar perovskite solar

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cells based on BSO and TiO2 ETL.

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Fig. 11. Typical steady-state PL spectra (a) and TR-PL decay spectra (b) of the perovskite films deposited on BSO and TiO2 films, respectively.

Fig. 12. Nyquist plots of BSO and TiO2 based PSCs under 1 sun light illumination conditions (a) and dark conditions (c); and the fitted result (solid line) is fitted to experimental data (symbols)

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using the equivalent circuit (b); potential dependence of recombination resistance (Rrec) from

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impedance spectroscopy measurements (d) for BSO and TiO2 based solar cells.

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Fig. 10

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Fig. 11

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Fig. 12

ACCEPTED MANUSCRIPT Table Captions Table 1. Statistical photovoltaic parameters of BSO-based planar PSCs extracted from J-V measurements under 1 sun illumination (AM 1.5G, 100 mW/cm2). Best results of each type of

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devices prepared with various PbI2 films are listed in bracket.

ACCEPTED MANUSCRIPT Table 1 Jsc a (mA/cm2)

Voc b (V)

FF c

PCE d (%)

Rsh e (Ω·cm2)

Rse (Ω·cm2)

BSO-1

3.99±0.86 (4.38)

0.475±0.176 (0.596)

0.248±0.047 (0.364)

0.44±0.21 (0.80)

73.5±27.4

101.7± 51.1

BSO-2

15.27±1.56 (17.51)

0.793±0.062 (0.746)

0.533±0.063 (0.675)

6.45±1.15 (8.82)

533.7±89.4

14.1±1.59

BSO-3

15.68±1.90 (17.45)

0.984±0.005 (0.986)

0.628±0.038 (0.637)

9.65±1.06 (10.96)

BSO-4

12.36±0.02 (12.05)

0.991±0.055 (1.020)

0.637±0.062 (0.709)

7.77±0.61 (8.72)

efficiency; e Rsh and

e

b

618.9±165.0

13.3±2.1

360.0±117.5

19.5±2.3

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Open-circuit photovoltage;

c

Fill factor;

d

Power conversion

Rs represent shunt resistance and series resistance, respectively, derived

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BSO films

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from J−V curve.

ACCEPTED MANUSCRIPT Highlights

Ternary oxide BaSnO3 have been synthesized by a facile peroxide-precipitate route

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BaSnO3 was first used as planar electron-transporting layer in PSCs

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The influence of the quality and thickness of the BaSnO3 layer was studied

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Enhanced PCE of cells fabricated by employing optimized-BSO layer was

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obtained